In general pulsed Doppler radars transmit pulses of coherent radio frequency into a region under observation. These pulses are reflected by the objects in the region and each object changes the frequency of the reflected pulse in dependence on the velocity of the object relative to the radar, as governed by the Doppler principal. Generally, objects which are moving relative to the ground in the region of observation are targets, and signals reflected from the ground, i.e., trees waving in the wind, waves in water, clouds, etc. are commonly referred as clutter.
Radars of aircraft or missiles flying at relatively low altitudes encounter severe reflection of the radar signals off of the ground environment and off of such weather conditions as clouds, fog, and drizzle. These clutter signals further complicate the synthesis of velocity and ranging information because of radar response ambiguity peaks. Conventional prior art pulse modulation and prior frequency modulation radars have ambiguity response characteristics comprised of a multiplicity of unwanted ambiguity response peaks on their frequency and time axes which makes them sensitive to clutter and high level corrupting signals. U.S. Pat. No. 3,883,871, entitled "Method and Modulation System for Ambiguity Reduction in Pulse Radar", issued May 13, 1975, discloses a modulation system for pulsed radar wherein the modulated signal rotates the time-frequency plane of the pulse modulation ambiguity response pattern of the radar with respect to the amplitude axis so that unwanted ambiguities are shifted off of the time and frequency axis to facilitate velocity and range measurement. This method and apparatus greatly reduces the ambiguities in the system but, while the effects of clutter are greatly reduced, clutter is still a problem.
U.S. Pat. No. 4,119,966, entitled "Clutter Discriminating Apparatus for Use with Pulsed Doppler Radar Systems and the Like", issued Oct. 10, 1978, describes apparatus for differentiating desired targets from clutter in the return of a pulsed Doppler radar. For certain target velocities and aspect angles, the apparatus described in both of the above referenced patents and all other phase and frequency coded modulation pulsed radar the target return is allowed to be obliterated by radar clutter. For typical target velocities the number of aspect angles that are obscured by clutter is between 10 and 20. In the prior art, the radar designer has to assign some likelihood to the different types of target encounters with which he is concerned. He then selects his modulation parameters to minimize the number of ambiguities in the most likely target geometries. Having done this he is forced to accept the inability to separate the target from the clutter at the obliterated velocities and aspect angles.